fix: global scope map variables from function returns get correct type

[cs01]

Summary

• const m = makeMap() at global scope where makeMap() returns Map<string, string> was incorrectly typed as %Map* (numeric map) instead of %StringMap*, causing invalid LLVM IR
• Root cause: the map sub-type detection only checked stmt.declaredType and MapNode.keyType, but when the value is a function call, neither is set. Now also extracts key/value types from the resolved expression type
• Same fix applied for pointer maps (non-string key types)

Test plan

• New fixture maps/global-map-return.ts — function returning Map<string, string> used at global scope
• All 616 tests pass
• Self-hosting verification passes (Stage 0 + Stage 1)

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